1. Technical Field
The present invention relates to antenna structures provided in wireless communication apparatuses, such as cellular phones, and to wireless communication apparatuses including the antenna structures.
2. Background Art
FIG. 13a is a schematic perspective view showing an example of an antenna structure (e.g., refer to Patent Document 1). The antenna structure 40 includes a dielectric base 41 having a rectangular parallelepiped shape, and a ground electrode 42 is formed on the bottom surface of the dielectric base 41. Furthermore, on the top surface of the dielectric base 41, a driven radiating or feeding electrode 43 and a parasitic radiating or non-feeding electrode 44 are provided adjacent to each other, separated by a slit s1. On a side surface of the dielectric base 41, a connecting electrode 45 and a connecting electrode 46, spaced from each other. The connecting electrode 45 serves to electrically connect the driven radiating electrode 43 and the ground electrode 42. The connecting electrode 46 serves to electrically connect the parasitic radiating electrode 44 and the ground electrode 42.
On a side surface of the dielectric base 41 opposing the surface on which the connecting electrodes 45 and 46 are formed, a feeding electrode 47 for the driven radiating electrode is formed, and a frequency controlling electrode 48 is also formed. An upper end of the feeding electrode 47 is provided with a space from the driven radiating electrode 43 so as to form a capacitor with the driven radiating electrode 43. A lower end of the feeding electrode 47 is formed so as to extend to the bottom surface of the dielectric base 41. The lower end of the feeding electrode 47 is provided with a space from the ground electrode 42, and the lower end of the feeding electrode 47 is electrically connected to, for example, a high-frequency circuit 50 for wireless communication provided in a wireless communication apparatus. An upper end of the frequency controlling electrode 48 is provided with a space from the driven radiating electrode 43 and with a space from the parasitic radiating electrode 44 so as to form capacitors C1 and C2 with the driven radiating electrode 43 and the parasitic radiating electrode 44, respectively. A lower end of the frequency controlling electrode is formed so as to extend to the bottom surface of the dielectric base 41. The lower end of the frequency controlling electrode 48 is provided with a space from the ground electrode 42. Furthermore, the lower end of the frequency controlling electrode 48 is grounded via switching means 51, for example, to the ground of a wireless communication apparatus.
In the antenna structure 40 shown in FIG. 13a, for example, when a signal to send has been supplied from the high-frequency circuit 50 for wireless communication to the feeding electrode 47, through capacitive coupling between the feeding electrode and the driven radiating electrode 43, the signal to send is transmitted from the feeding electrode 47 to the driven radiating electrode 43, whereby the driven radiating electrode 43 resonates according to the signal to send. Furthermore, the signal to send is also transmitted to the parasitic radiating electrode 44 through electromagnetic coupling between the driven radiating electrode 43 and the parasitic radiating electrode 44, whereby the parasitic radiating electrode also resonates. In the antenna structure 40, the space s1 between the driven radiating electrode 43 and the parasitic radiating electrode 44 and other factors are designed so that the resonance of the driven radiating electrode 43 and the resonance of the parasitic radiating electrode 44 cause multiple resonance.
The resonant operation (multiple resonant operation) of the driven radiating electrode 43 and the parasitic radiating electrode 44 is an antenna operation that sends the signal to send wirelessly to the outside. Furthermore, when a signal from the outside has reached the driven radiating electrode 43 and the parasitic radiating electrode 44, the driven radiating electrode 43 and the parasitic radiating electrode 44 resonate according to the received signal, whereby the received signal is transmitted from the driven radiating electrode 43 to the feeding electrode 47 and further to the high-frequency circuit 50 for wireless communication. The resonant operation of the driven radiating electrode 43 and the parasitic radiating electrode 44 according to the wireless communication signal from the outside, described above, is an antenna operation for reception.
In the antenna structure 40, the frequency controlling electrode 48 forms capacitors individually with the driven radiating electrode 43 and the parasitic radiating electrode 44, and the frequency controlling electrode 48 is grounded via the switching means 51. With this configuration, in the antenna structure 40, it is possible to switch the resonant frequency bands of the driven radiating electrode 43 and the parasitic radiating electrode 44 as described below. For example, let it be supposed that when the switching means 51 is OFF so that the frequency controlling electrode 48 is not grounded, for example, the driven radiating electrode 43 has a resonant frequency band indicated by a dotted line A having a resonant frequency f1 shown in FIG. 13b, the parasitic radiating electrode 44 has a resonant frequency band indicated by a chain line B having a resonant frequency f2 shown in FIG. 13b, and the driven radiating electrode 43 and the parasitic radiating electrode 44 cause multiple resonance as indicated by a solid line a in FIG. 13b. 
On the other hand, when the switching means 51 becomes ON so that the frequency controlling electrode 48 is grounded, capacitors are formed with the ground between the driven radiating electrode 43 and the frequency controlling electrode 48 and the parasitic radiating electrode 44 and the frequency controlling electrode 48. Thus, a capacitance with the ground is loaded to the driven radiating electrode 43, and also a capacitance with the ground is loaded to the parasitic radiating electrode 44.
FIG. 13c shows an equivalent circuit of the driven radiating electrode 43 by solid lines. Since the resonant operation of the driven radiating electrode 43 is an LC resonance of an inductance component L and a capacitance component C of the driven radiating electrode 43, shown in FIG. 13c, the resonant frequency F of the driven radiating electrode 43 is proportional to 1/√(LC) (F∝1/√(LC)). This similarly applies to the resonant frequency of the parasitic radiating electrode 44. Thus, when the switching means 51 becomes ON so that capacitances with the ground are loaded to the driven radiating electrode 43 and the parasitic radiating electrode 44 by the frequency loading electrode 48, the capacitance components of the driven radiating electrode 43 and the parasitic radiating electrode 44 increase, so that the resonant frequencies of the driven radiating electrode 43 and the parasitic radiating electrode 44 become lower. Thus, when the switching means 51 is switched from OFF to ON, for example, the resonant frequency of the driven radiating electrode 43 is switched from the frequency f1 to a frequency f1′, and for example, the resonant frequency of the parasitic radiating electrode 44 is switched from the frequency f2 to a frequency f2′. Thus, the multiple resonance by the driven radiating electrode 43 and the parasitic radiating electrode 44 is switched from the state indicated by the solid line α the state indicated by a solid line β in FIG. 13b. 
In this antenna structure, when the switching means 51 is OFF, the frequency bands for wireless communication by antenna operations of the driven radiating electrode 43 and the parasitic radiating electrode 44 fall in a frequency range of, for example, a frequency fm to a frequency fn shown in FIG. 13b. On the other hand, when the switching means 51 is ON, the frequency bands for wireless communication by antenna operations of the driven radiating electrode 43 and the parasitic radiating electrode 44 are switched, for example, to a frequency range from a frequency fm′ to a frequency fn′ shown in FIG. 13b. 
Thus, for example, in a case where the configuration for frequency switching described above is provided, the antenna structure 40 can support wireless communication in the frequency range of, for example, the frequency fm′ to the frequency fn′. That is, it is possible to increase the frequency band of the antenna structure 40. This is in contrast to a case where no configuration for frequency switching by the frequency controlling electrode 48 is provided, in which the frequency bands for wireless communication by antenna operations of the driven radiating electrode 43 and the parasitic radiating electrode 44 fall only in the frequency range of, for example, the frequency fm to the frequency fn.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2001-168634
Patent Document 2: Japanese Unexamined Patent Application Publication No. 2005-150937